There are many examples of radio communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, utilise OFDM. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period, which can be longer than a coherence time of the radio channel. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols.
Multiple-Input Single-Output (MISO) is a technique which can be used in combination with Alamouti encoding to improve the robustness with which data can be received. Alamouti encoding forms pairs of data symbols, which are then differently encoded for communication via each of the channels. MISO/Alamouti encoding can therefore be used in combination to form robust and effective communication of data, and in particular when used with OFDM. However, in order to decode Alamouti encoded data cells it is necessary to form an estimate of each channel through which each version of the OFDM symbols of the MISO system have passed. To this end, it is known to include pilot data which is conveyed by sub-carriers of the OFDM symbols assigned for this purpose. For example, the DVB-T2 system has been designed to include pilot patterns to allow the system to be deployed in a MISO mode and to utilise Alamouti encoding, as disclosed in “ETSI EN 300 755, “Digital Video Broadcasting (DVB): Frame structure, channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”, May 2008.
Detecting and recovering data from OFDM symbols which have been transmitted in accordance with a MISO/Alamouti encoding system represents a technical problem, in particular when the receiver is required to generate an estimate of each of the first and second channels via which the OFDM symbols are received.